1. Field of the Invention
The present invention relates to a liquid jet recording apparatus.
More specifically, the present invention relates to a liquid jet recording apparatus using a full-line type recording head that has a discharge port array with a length substantially equal to the width of a recording medium and that carries out image recording onto substantially the whole surface of a recording medium by relatively scanning the recording medium with the recording head for only one time.
Even more specifically, the present invention relates to a liquid jet recording apparatus that supplies ink to the head from a second tank that is different from a liquid reserve tank.
2. Description of the Related Art
Conventionally, in a liquid jet head that discharges an ink droplet by utilizing thermal energy such as heat generated by a heater and the like, some of the thermal energy generated by the heater is discharged to the outside of the head by the discharge of the ink droplet.
However, in some cases, the remaining thermal energy is stored in the liquid jet head, so that the temperature of the liquid jet head is raised.
Due to the rising of the temperature of the liquid jet head, the ink in the head is more intensely heated.
On the other hand, the viscosity of the ink is reduced as the temperature of the head is raised. Thus, in the case of the ink with reduced viscosity, a larger amount of ink is discharged even when the energy of the same level as compared to the case of an ink having a viscosity of a normal level is applied.
In addition, the temperature of the head is raised as the number of copies to be printed increases. Therefore, there is a problem such that in this case, the density of an image is gradually increased.
In addition, when the temperature of the head is raised, air dissolving in the ink supplied to the head is separated out. Then, the separated-out air is accumulated, and as a result, the ink is not supplied to the discharge ports in a sufficient amount. Thus, there occurs phenomena of an instability of a discharge operation and a discharge failure in the worst case.
An effect of the rise in the temperature appears more remarkable in a recording apparatus using a long full-line type head that has a large number of nozzles distributed in a high density and that carries out recording onto substantially the whole surface of the recording medium by a single relative movement of the head and the recording medium.
In this type of conventional liquid jet recording apparatus, an excellent image is obtained by cooling the full-line type head in order to prevent the temperature rise of the head.
In recent years, the trend for higher definition and higher printing speed of a liquid jet recording apparatus is getting more and more developed. With regard to resolution, for example, the trend is transited from 1,200 dpi to 2,400 dpi, and then to 4,800 dpi. As is known from this, a product of a higher resolution is brought out to the market year by year.
In accordance with the improvement of the resolution, an ink droplet discharged by one discharge operation is getting smaller and smaller in size. In this regard, at present times, there is a product that is capable of discharging a very small ink droplet of 2 pl.
However, as the resolution becomes higher, it is necessary to make a discharging frequency higher in accordance with the higher resolution. Thereby, the temperature rise of the liquid jet head becomes much greater.
In the case of a recording apparatus using a full-line type liquid jet head that carries out printing by only one relative movement (single pass) of a recording medium and the recording head, an image is finally formed by one relative movement of the recording medium and the recording head.
However, there is a problem such that it is impossible to decrease the number of ink discharge nozzles used at the same time to a submultiple of the number of nozzles by carrying out the image formation by distributing the operation into multiple passes as in the case of a serial type recording apparatus that forms an image by reciprocating movement of the head.
In a case where the recording by a single pass by the full-line type head is finally carried out, it is necessary to use the whole portion of the head in the single pass. Therefore, the rise in the temperature of the head caused by continuous printing becomes more remarkable.
Practically, in order to implement high speed printing by which 60 sheets of A4 size paper are fed per one minute in the direction of a shorter side of the paper (in a horizontal direction), a discharge frequency of the head of 16 kHz is necessary for the resolution of 1,200 dpi.
When the resolution is doubled to 2,400 dpi, a discharge frequency of the head of 32 kHz is necessary. In other words, when the resolution becomes high and the size of the ink droplet is made smaller in accordance with the higher resolution, it is necessary to increase the number of ink droplets to be discharged per unit area.
Therefore, in the conventional recording apparatus, in order to implement high speed printing, it is necessary to increase the ink discharge frequency to a large extent. As a result, the intense rise in the temperature of the head is caused.
In the recording apparatus using the full-line type head, the head itself is long, and accordingly, a temperature distribution in the head is likely to occur.
Especially, in the vicinity of an ink supply port through which ink flows into the head in order to replenish ink in an amount equal to an amount of the discharged ink, the ink of a low temperature flows in, and therefore, the temperature is relatively lowered. On the other hand, in a portion distant from the ink supply port, the ink that is heated to some extent in the head is supplied to the nozzle, and therefore, the temperature is raised. Thus, there is a problem such that the temperature distribution is caused in the same head, and the printing density becomes uneven.
In a recording apparatus disclosed in Japanese Patent Application Laid-Open No. 2000-255048, as shown in FIG. 14 and FIG. 15, one end of an ink supply path to a head 1 is connected to an ink cartridge 3, and the other end of the ink supply path is connected to a sub-tank 4.
A pipe-like circuit in the head 1 is filled with ink. When an amount of consumed ink reaches a given amount during printing, the ink is automatically supplied from the ink cartridge 3 to the head 1, and the ink passes through the pipe-like circuit in the head 1 and is then returned to the sub-tank 4.
However, a timing at which the ink is supplied to the head 1 is determined in advance. In a case where a continuous printing at a high resolution is carried out by a full multihead, the ink supply for offsetting the rise in the temperature cannot be implemented, and it is not possible to carry out a sufficient cooling of the head.
In addition, ink is directly supplied from the ink cartridge 3 into the head 1, and, thereby, a liquid level of the ink cartridge 3 fluctuates. Therefore, the recording apparatus of this type is liable to be directly affected by the fluctuation of a head difference between the liquid levels of the ink cartridge 3 and an ink discharge section.
In a recording apparatus disclosed in U.S. Pat. No. 4,896,172, which is a second conventional example, a liquid jet recording apparatus provided with a switching unit configured to be capable of switching between an ink supply system and an ink circulation system is discussed.
As shown in FIG. 16 and FIG. 17, a heating element 35, a common liquid chamber 33, a recording liquid supply port 36A, and a second supply port 36B used at the time of the circulation of the liquid are provided on an element substrate.
A recording head 31 includes a liquid path 34 and an orifice 32.
Further, the head 31 is provided with a cooling chamber 39 formed on the element substrate in a manner opposed to the liquid path 34, a temperature sensor 38, an ink supply port 310A for supplying the liquid to the cooling chamber 39, and a liquid return port 310B.
In the recording apparatus disclosed in U.S. Pat. No. 4,896,172, the rise in the temperature in the head 31 is detected by the temperature sensor 38. If the temperature is above a certain given level as a result of the detection, the switching unit switches the ink supply path. That is, the ink is circulated between a recording liquid reserve tank 37 and the head 31 in order to cool the head 31, as indicated by full line arrows shown in FIG. 18.
The liquid circulation path is constituted by valves 314, 315A, and 315B, pumps 313, 314A, 325A, and 325B, the recording liquid reserve tank 37, liquid supply paths 312B and 312A, and liquid return paths 311A and 311B.
However, in the configuration of the recording apparatus disclosed in U.S. Pat. No. 4,896,172, it is necessary to switch the ink supply path by the switching unit in carrying out the cooling of the head.
That is, in a case where continuous printing is carried out by a liquid jet recording apparatus using a full-line type liquid jet head capable of high speed printing, there is a problem such that a capacity of cooling the head is not high enough.